Twisted nematic (TN) liquid crystal displays (LCD) have invaded all aspects of modern life. They generally rely on the light switching capabilities of pixel cells having a sandwich with a space containing a liquid crystal compound surrounded by electrodes, the sandwich being effectively placed between polarizing filters.
A typical TN-LCD cell has affects incident light according to FIG. 1. Note that response is bipolar—light transmission depends on the absolute value of the difference between electrode voltages, not on the polarity of the difference. The zone 50 of full light transmission may be interchanged with the zone 52 of black or opaqueness by rotating one of the polarizing filters, and for intermediate voltages 54 grey is obtained.
Reflective-mode LCOS (Liquid Crystal On Silicon) displays have one transparent electrode, typically formed of indium-tin-oxide (ITO) and one reflective electrode, typically formed of aluminum (AL), in alternative embodiments the reflective electrode may be formed of other reflective metals. In these displays, a single polarizing filter in front of the ITO electrode will allow incident light to enter and be reflected by the reflective electrode to leave the display through the ITO electrode with no voltage applied, but with a voltage between ITO and AL electrode polarization of passing light is rotated so light incident where the voltage is applied is blocked.
In some displays, it is desirable to alternate between operation in the negative-black region 56 with operation in the positive-black region 58 to better stabilize ions that accumulate in liquid crystals as they age.
In some displays, switching of polarity between the ITO 104 (FIG. 2) and AL or pixel 102 electrodes causes artifacts on the screen if the rise and fall times of signals applied to ITO and AL electrodes are not well matched because, although the relative voltages between ITO and AL electrodes start 106 and end 108 at the same value, they effectively “glitch,” 110 to an unintended value across the liquid crystal pixel element during transitions. The liquid crystal pixel may respond to this glitch by changing the amount of light it passes.
Other problems may also afflict liquid crystal displays. For example, ionized salts may contaminate the LCD cell, or ionized salts may form as liquid crystals break down with age. When a salt-containing LCD cell first receives a voltage 150 (FIG. 3), the full electric field appears across the liquid crystals, but as ions are attracted to the electrodes, a space-charge 152 develops that eventually shields 154 the liquid crystals from applied voltage while voltage is applied, reducing display contrast. Further, when pixel data changes, the space charge can provide an image-memory effect 156.